Treatment of sleep disordered breathing (SDB), such as obstructive sleep apnea (OSA), by continuous positive airway pressure (CPAP) flow generator systems involves the continuous delivery of air (or other breathable gas) pressurized above atmospheric pressure to the airways of a human or other mammalian patient via a conduit and a mask. Typically, the mask fits over the mouth and/or nose of the patient. Pressurized air flows to the mask and to the airways of the patient via the nose and/or mouth. As the patient exhales, carbon dioxide gas may collect in the mask. A washout vent in the mask or conduit discharges the exhaled gas from the mask atmosphere.
The washout vent is normally located in the mask or near the mask in the gas delivery conduit coupled to the mask. The washout of gas through the vent to the atmosphere removes exhaled gases to prevent carbon dioxide build-up, and hence “rebreathing”, which represent a health risk to the mask wearer. Adequate gas washout is achieved by selecting a vent size and configuration that allows a minimum safe washout flow at a low operating CPAP pressure, which typically can be as low as 4 cm H2O for adults and 2 cm H2O for children.
Noise is a significant issue in CPAP treatment for the patient and/or the patient's bed partner. Excessive noise can lead to patients being non-compliant with the CPAP therapy. One source of noise is the exhaust through the vent in the mask or conduit. The flow of gas through the vent creates noise as it exits to and interacts with the atmosphere. Noise can adversely affect patient and bed-partner comfort, depending on both the magnitude and character of the noise. Further, bi-level gas delivery regimes tend to generate more noise than do constant level gas delivery regimes. This is thought to be due to the extra turbulence created by the gas accelerating and decelerating as it cycles between relatively low and relatively high pressures in the bi-level gas delivery systems.
The washout vents may offer a generally fixed impedance to air flow (for example having a generally unchanging geometry with time) or a variable impedance. In a fixed vent design, the vent flow increases with mask pressure, such that the vent flow may be adequate at a low pressure and excessive at high pressure. The variations in flow through fixed vent can lead to noise. Fixed vents are generally simple and inexpensive to make and operate. A variable vent design could provide a constant or near constant vent flow across a range of mask pressures having the advantage that noise will not increase as mask pressure is increased. This may lead to lower vent flow at high pressures which may in turn lead to less noise. However variable vents suffer from difficulties in manufacture, assembly, consistency, cleaning and usability.
Fixed gas vents are known that have relatively low noise levels, which levels may be as low as 30 dBA at a therapy (mask) pressure of 12 cm H2O. Such vents include, for example, the Resumed MIRAGE™ mask (disclosed in U.S. Pat. No. 6,561,190), the ResMed ULTRA MIRAGE™ mask (disclosed in U.S. Pat. No. 6,691,707), the ResMed VISTA™ mask (disclosed in US Published Patent application 2003/0196657), the ResMed ACTIVA™ mask that includes an elbow with a vent (disclosed in International Patent Application PCT/AU03/01162 published as WO 2004/022147) and the ResMed MERIDIAN™ disposable nasal mask that includes an elbow incorporating a vent (disclosed in International Patent Application PCT/AU2004/000563). The contents of all of these patents and patent applications are incorporated herein by reference in their entireties.
Various quiet vents are known that provide noise levels generally in the range of 25 dBA or less which makes the vent noise difficult to distinguish from transmitted flow generator noise or general background noise. Examples of quiet vents are disclosed in U.S. Pat. No. 6,581,594 and the Weinmann sintered vent. At least the sintered vent is believed to suffer from poor manufacturability, durability, blockage, humidification, sterility/bacterial growth, and/or cleanability.
There is a long felt and continuing need for quiet gas vents for masks and conduits, that are relatively inexpensive, simple in their construction and easy to maintain. Reducing the noise of gas being exhausted from a mask or conduit can significantly improve the user friendliness of the CPAP treatment. Providing a simple and easy to use low-noise vent can reduce the cost of CPAP treatments and thereby assist in making the treatment more affordable to patients suffering from SDB.